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clip at the back of the combustion\^,^rrd^cautiouslyremovimr
the stopper from the potash apparatus^ 'S^^f^^^r^^i
way tap from its socket for a moment. Sx^^n"^* '*• — ^ * ".,

The Combustion.—Turn on the oxygen ^TaSjusTthe rate
of flow through the apparatus by means of the screw-clip' so
that • 2 or 3 bubbles a second pass through the potash bulbs
Throw back the tiles if closed, and light the burners under the
front layer of copper oxide to within 10 cm. (4 in.) of the boat
and. also 2 or 3 burners under the spiral behind the boat, but
not within 5 cm. (2 in.) of the boat. Turn up the gas slowly to
avoid cracking the tube and in a minute or two, when the tube
is thoroughly warmed, close the tiles over the lighted burners
and heat to a dull red heat. A vivid red heat during the
combustion is not only unnecessary, but undesirable, as the
glass is apt to soften and be distorted and even to blow out and

FIG. 10.
become perforated. A combustion tuoe carefully handled
should last indefinitely. When the copper oxide is red hot,
turn on the burners very gradually from the spiral towards the
boat, but do not close the two pairs of tiles over the boat until
the combustion is nearly terminated and the burners are all
lighted. The first indication of the substance burning is the
appearance of a film of moisture at the front end of the
combustion tube and an increase in the speed of the bubbles
passing through the potash apparatus. The front end of the
tube, which should project 4 to 5 cm. (i-J- to 2 in.) from the
furnace, must be kept sufficiently hot to prevent moisture
permanently condensing there ; but it must never be allowed to
become so hot that there is any risk of the cork being burnt,
and it should always be possible to place the finger and thumb
found the part of the tube where the cork is inserted, A screen
^made 4rom a square piece of asbestos board, with a slit in it